Most small engine applications today require the use of two-stroke or four-stroke engines. In a four-stroke engine, the cycle of the engine is completed in two revolutions of the engine crankshaft. The first stroke of the cycle is the inlet stroke, followed by the compression stroke, the power stroke, and then the exhaust stroke. In contrast, two-stroke engines perform the compression and power stroke of the four-stroke cycle at the same time as the inlet and exhaust strokes, respectively thus requiring only one revolution of the engine crankshaft.
The different mechanical aspects of a two-stroke versus a four-stroke engine have dictated differing constructions in the carburetors of prior engines. For example, two-stroke engines typically use a diaphragm-type fuel pump to supply fuel to the engine. The use of a fuel pump is required for multiple position use of the engine to assure constant fuel supply to the engine. In two-stroke engines, the diaphragm fuel pump is actuated by an air pulse signal created in the crankcase by the revolution of the engine. A pressure pulse is created on the "down" stroke of the engine piston and a corresponding vacuum pulse is created on the "up" stroke of the engine piston. The pressure and vacuum pulse is then fed from the crankcase to the fuel pump, causing oscillation of the fuel pump diaphragm, thus drawing fuel from the engine fuel tank to a constant fuel chamber and then into the air intake passage. Upon entry into the air intake passage, the fuel is atomized and mixed with air to be fed into the engine cylinder for firing.
Although effective with two-stroke configurations, diaphragm-type fuel pumps have not proved as effective for four-stroke engine applications. A four-stroke engine does not have a pressure/vacuum signal in the crankcase capable of driving a diaphragm-type fuel pump. Most four-stroke engines instead take the vacuum signal originating in the air intake passage and feed this signal to one side of the diaphragm. There is no corresponding pressure signal to cause oscillation of the diaphragm pump; the diaphragm is instead subjected to a neutral pressure signal followed by a vacuum signal. The neutral pressure signal in conjunction with the vacuum signal from the air intake passage is not sufficient by themselves to create oscillation of the diaphragm. Without sufficient oscillation of the diaphragm, the fuel pump cannot effectively draw fuel from the fuel tank into the carburetor system.
An alternate means is thus necessary in four-stroke applications to drive the pump and provide the required fuel for starting and normal operation of the engine. Instead of providing a pressure signal to the fuel pump diaphragm, four-stroke engines are configured with a coil spring device (see FIG. 1) to aid in the return of the diaphragm to the neutral position during the neutral time of the engine cycle. Although this configuration is not as effective as the plus-minus signal present in two-stroke engines, it has generally been the only means available to drive diaphragm-type fuel pumps.
A number of problems have been encountered in traditional coil spring fuel diaphragms. The use of a vacuum/spring arrangement to drive the fuel pump is not desirable because it results in increased wear on the diaphragm structure. The coil spring is in constant frictional contact with the material of the diaphragm during engine operation. Fuel pump diaphragms are weakened by a combination of degradation from contact with fuel and frictional contact from the spring coil return means, often resulting in torn, damaged, or otherwise ineffective diaphragms which must be repaired or replaced. In order to combat the wear problem caused by the use of a spring return, prior fuel diaphragms were often made of rubber which is sturdier and more wear resistant than many of the available materials suitable to this application. However, the use of rubber has become less desirable as it is a material that is not suited for use with the more advanced alcohol fuels now being used in two-cycle and four-cycle engines. Alcohol quickly degrades rubber, again causing breakdown and tearing of the diaphragm and additional expense and delay is caused by having to repair or replace the damaged diaphragm.
In order to prevent premature breakdown and tearing of the diaphragm caused by exposure to alcohol, a more alcohol-resistant material such as MYLAR.TM., PETP or LUMIRROR.TM. can be chosen. Although these materials prevent the breakdown of the diaphragm due to alcohol exposure, they are considerably less wear resistant than rubber and wear out more quickly from the action of the spring.
It can be seen that a number of unique problems have been encountered in the design and construction of diaphragm-type fuel pumps for four-cycle engines. Prior art diaphragms which purport to overcome these problems have been unable to provide the wear resistant qualities required by the use of a spring return mechanism with the alcohol resistant qualities mandated by today's alcohol fuels.
It would thus be desirable to provide a diaphragm-type fuel carburetor which simply and effectively eliminates the aforementioned difficulties. The fuel pump would preferably eliminate the necessity for a high-wear spring arrangement which requires the use of rubber diaphragm to provide sufficient wear resistance. It would be desirable to have a diaphragmtype fuel pump for small engine applications which can be made from alcohol resistant materials such as polyethylene terephthalate (PETP), MYLAR.TM. or LUMIRROR.TM.. The fuel pump should also provide a stronger, more stable fuel pressure than provided by a spring-type fuel pump diaphragm.
An improved diaphragm-type fuel pump should preferably be suitable for use with small engines having a fuel pump as an integral part of the carburetor as well as for engines having a fuel pump that is a separate assembly from the carburetor. Such a device should also be suitable for both two-stroke and four-stroke engines. It would be desirable for an improved diaphragm-type carburetor to meet the above objectives and goals without adding significant complication or expense to the carburetor assembly.